1. Field of the Invention
The present invention relates to the field of optical fibers, more particularly active optical fibers for Raman amplification. An optimized fiber of this kind may be used to produce a tunable multiwavelength laser source operating by virtue of stimulated Raman emission in the fiber.
2. Description of the Related Art
“Les télécommunications par fibers optiques” [“Optical fiber telecommunications”] by I. and M. Joindot, Dunod, 1996, defines the Raman effect as photon-phonon coupling between an electromagnetic field and mechanical vibration of a medium. The Raman effect corresponds to coupling with optical phonons. Optical phonons correspond to vibrations internal to the molecular structures of the material. It is a nonlinear effect in which a pump photon is absorbed and a signal photon of lower energy is emitted from the material at the same time as a phonon.
Each material has a spontaneous Raman emission spectrum at given wavelengths, i.e. behaves like a set of oscillators each at a given vibration frequency. An emission spectrum characteristic of the material may therefore be established, with intensity spikes at given wavelengths known as Stokes lines. Glass being a disorderly amorphous material, its Raman spectrum is characterized by a large number of wavelengths forming a continuous spectrum over a wide band of frequencies.
The intensity of Raman emission increases with the input power applied to the material and becomes significant at a given power called the threshold. Thus by stimulating the material at a high optical power, for example using a pump laser, the intensity peak corresponding to spontaneous Raman emission can reach 100% of the intensity of the pump signal at a given wavelength. By exploiting the Raman emission of a material, it is therefore possible to amplify an optical signal strongly by passing it through said material when stimulated by an optical pump signal. An optical signal is also amplified if it has an optical wavelength at a given offset relative to the pump wavelength; in other words the wavelength of the Raman emission peak depends on the wavelength of the pump signal. Thus a Stokes shift is defined representative of the difference between the optical frequency in question and the pump optical frequency.
By feeding an amplified optical signal into the Raman amplification medium several times, it is possible to amplify and shift the optical signal progressively to obtain required optical power and wavelength characteristics.
Furthermore, a laser may be produced by associating the Raman amplification medium with appropriate Fabry-Perot cavities.
Raman amplifiers and Raman fiber lasers are therefore of great benefit in optical transmission system applications.
Modules are generally used that combine a plurality of pumps consisting of semiconductor diodes to pump Raman amplifiers. Multipump modules are costly, however, and consume a great deal of energy. Also, multipump modules employ a plurality of polarization-multiplexed diodes. Thus failure of one diode depolarizes the source. It is therefore beneficial to replace multipump modules with a Raman fiber laser that may be tuned to multiple wavelengths.
Most Raman amplifiers or Raman fiber lasers use fibers based on silica, germanosilicate or phosphosilicate. The Raman spectra of silica and germanosilicate comprise a single Raman emission band whose width yields shifts that do not exceed 100 nanometers (nm) when the material is subjected to a pump signal at 1.5 micrometers (μm). Fibers based on phosphosilicate have a Raman emission spectrum with two peaks yielding, when the material is subjected to a pump signal at 1.4 μm, a first shift that does not exceed 100 nm and corresponds to the shift produced by the silica and an additional shift of 300 nm.
Furthermore, a tunable Raman fiber laser is obtained by providing selection means at the output of the above kind of Raman fiber, for example by means of Bragg gratings. One such application is described in particular in “Six output wavelength Raman fiber laser for Raman amplification” by F. Leplingard et al., Electronics Letters, Aug. 1, 2002, Vol. 38, No. 16, pp. 886-887.
However, Raman amplifying fibers based on silica or germanosilicate have the drawbacks of a narrow amplification peak and the necessity to provide a large number of Bragg gratings to obtain a tunable wavelength laser. Raman amplification fibers based on phosphosilicate have the advantage of two widely spaced amplification peaks, which reduces the number of Bragg gratings, but their amplification gain is lower than that of Raman fibers based on silica or germanosilicate and the second amplification peak is very narrow.
If a multiwavelength laser is modeled on the basis of a Raman amplification fiber, the interactions between the peaks are significant and lead to gain differences that make the wavelengths emitted by the Raman fiber laser interdependent, which complicates the design of the laser.